this is in Mongoid 2ish.

fails.rb

require 'mongoid'

class Foo
  include Mongoid::Document
  field :bar, :type => String

  validates_uniquness_of :bar
end

foo = Foo.new
foo.valid? 
# => MONGODB (0ms) foo['$cmd'].find({:ismaster=>1}).limit(-1)                                │                                                                                       
# TypeError: db_name must be a string or symbol

works.rb

require 'mongoid'

class Foo
  include Mongoid::Document
  field :bar, :type => String
end

foo = Foo.new
foo.valid? # => true